Optical fibers have become mainstream as a transmission medium in the field of FTTHs (Fiber to the Home) and long-distance and medium-distance communication in the automotive field. Recent years have witnessed a growing need for high-speed transmission in which light is used also in short distances up to 1 m. Suitable for this region are optical waveguide-type optical wiring boards that have high-density wiring (for instance, with narrow pitch, high branching, more intersections and higher layer counts), are amenable to surface mounting and integration with electrical boards, and are bendable in a small radius, these properties not being achieved by optical fibers.
In broad terms, optical wiring boards have to meet the following two requirements. Firstly, optical wiring hoards are substitutes for printed wiring boards (PWBs). Secondly, optical wiring boards are substitutes for flexible printed boards (FPCs) that are used in hinges of small terminal devices.
In both types, low-speed signal transmission and electrical wiring are essential for operating, for instance, VCSELs (Vertical Cavity Surface Emitting Lasers), PDs (Photo Diodes) and ICs, which are light-emitting and receiving elements. Accordingly, the ideal configuration of an optical wiring board is herein an optical-electric composite wiring board that is a combination of an optical circuit and an electrical circuit (for instance, Patent literature 1).
In a photoelectric composite wiring board that combines an optical circuit and an electrical circuit, light must be inputted and outputted to/from by light-emitting element and a light-receiving element in the optical circuit. Accordingly, the optical circuit must be disposed on the surface layer of an electrical circuit multilayer substrate, and be disposed in such a manner that various chips can be mounted of the optical circuit.
In a case where an optical circuit is formed on an electrical circuit board that is provided in order to mount various types of chip, however, the already-formed electrical circuit that is provided in order to mount various types of chip becomes covered when there is used a non patternable optical circuit forming material that is applied over the entire surface and is then cured. Accordingly, mounting must take place after removal, by laser or by machining in a later step, of the material for the optical circuit that covers the electrical circuit. This translates into poorer productivity, and is thus problematic.
Alternatively, in a case where an optical circuit is formed on the electrical circuit multilayer substrate that has no electrical circuit, and that has formed therein only through-holes for electrical connection with an underlying layer, with no electrical circuit in the outermost layer, and where a further electrical circuit is to be formed, on the optical circuit, for chip mounting, then numerous through-holes must be formed for electrical connection with the underlying electrical circuit, after formation of the electrical circuit on the optical circuit, in particular when a non-patternable optical circuit forming material is used. This makes for very poor productivity.
A conceivable approach, other than the above methods, involves laminating, on an electrical circuit board, an optical circuit that is produced using a non-patternable optical circuit forming material. In this case, however, the electrical circuit and the optical circuit are produced in different processes, and, accordingly, an adhesive is required, and the optical circuit has to be aligned. Productivity is impaired in this instance as well.
In order to solve the above problems, it is useful to impart patternability, and adhesion by plating, to an optical wiring material itself. A conceivable method to that end involves adding, for instance, inorganic particles or rubber particles to the optical wiring material. It is however difficult to add large amounts of inorganic particles, rubber particles or the like to optical wiring materials, from which high transparency is demanded, and no such materials are known as yet.
Meanwhile, using a dry film for optical waveguides is a known feature in order to easily form an optical waveguide on a substrate. Hitherto known dry films for optical waveguides include dry films that have a base film and at least two photosensitive resin layers that exhibit different refractive indices after curing (for instance, Patent literature 2).
The dry film disclosed in Patent literature 2, however, does not address the issue of combining transparency and adhesion; also, the object of Patent document 2 is not imparting the surface layer of the dry film with adhesion towards plating.
Therefore, it is an object of the present invention to provide a dry film for optical waveguides that can be patterned and that allows realizing both transparency and plating adhesion, and to provide an optical waveguide and a photoelectric composite wiring board that use the dry film for optical waveguides, and a method for producing a photoelectric composite wiring board in which the dry film is used.